The myelin-associated glycoprotein (MAG) is a 100 K-dalton, integral membrane glycoprotein containing 30% carbohydrate that is in central and peripheral myelin sheaths. Recent research on its function during normal myelinogenesis and possible roles in the pathology of demyelinating diseases is reviewed. Emphasis is placed on studies supporting a role for MAG in glia-axon interactions. Also, a highly immunogenic carbohydrate antigen on MAG, that is shared with natural killer cells and binds IgM paraproteins associated with neuropathy, is considered in detail.